Hollow bodies produced from plastics material, such as, for example, canisters tanks and bottles, can be produced as a result of extrusion blow moulding. In this connection, first of all a hose is extruded from plasticized plastics material which is enclosed in a blow mould with a cavity which corresponds to the hollow body contour and is ballooned until it abuts against the cavity wall. Once a cooling time which is sufficient for achieving dimensional stability has been reached, the hollow body can be removed from the mould. A disadvantage of this method is the considerable expenditure on equipment, to which the clamping unit, the accumulator head, the blow mould, the regulations on wall thickness, the waste disposal device and the expanding device contribute. The different proportion of waste depending on the development of the hollow body has to be returned into the process again. A further disadvantage is the slower cooling of the inside of the hollow body as heat dissipation toward the blow air side is effected in a retarded manner.
German Patent Publication No. DE 1809990 A1 describes a method for producing plastics material containers where in a first step at least one pair of complementary hollow body parts are formed in one shot by injection moulding and in a second stage said hollow body parts are brought together and joined thus forming the cavity.
The hollow body parts remain in the outer moulds once the injection mould has been opened and are transferred with said outer moulds into an external joining station and there are joined non-detachably along their edges. As a result of injection moulding the hollow body halves, cooling is achieved quicker than in the case of blow moulding such that the cycle time is able to be reduced in a considerable manner. The wall thickness is predefined precisely by the cavity, and the proportion of waste is very small. In general, two mirror-symmetrical hollow body halves are formed which are joined along their edges to form the hollow body. In this case, the two halves can comprise differently realized edges, if this is necessary for the joining of the two halves. For example, overlaps or a reciprocal engagement in the form of a tongue and groove can be realized on the edges.
The processing of plastic sheets as a result of thermoforming where thermoplastic plastics materials are usually reshaped in the heated state, for example, deep-drawn in a vacuum, is additionally known. Sheets which are produced with nozzles for sheet or film extrusion of plastics materials and other plasticizable and hardenable materials are fundamental in this case. The material is usually heated up first of all and in the plastic state is pressed through the nozzle, between a nozzle bottom part and a nozzle top part, in order, after a cooling and hardening stage, to obtain a sheet which corresponds as precisely as possible to predefined dimensions.
Irrespective of from which method of production the parts of a large plastics material tank originate, the half-shells which are produced using the different methods have to be joined to one another in a process downstream. For example, hot plate welding is used for hollow containers which consist of two injection moulded plastics material half-shells. On account of the different tolerances of the half-shells, a joining tool must be provided in order to move the two container half-shells into a forced position. For the use of such a joining tool, an additional edge geometry, in particular a U-shaped profile, is provided along which the edges of the half-shells are welded.
In many applications it is desirable for the container half-shells to define as large a volume as possible. As the installation space in the vehicle is always limited, the use of a circumferential edge for the application of a joining tool is disadvantageous as the volume of the container is reduced here. The space formed by the edge is also unavailable for accommodating a liquid.
In order to fill out the installation space as well as possible, container half-shells are placed end to end on top of one another and are thus welded. The problem existing here is that the large-volume plastics material half-shells have high tolerances and bend and warp when being processed as a result of their own elasticity.